Peak clock: Fault injection into PLL-based systems via clock manipulation

Clock glitches are an inexpensive method to attack embedded systems. Usually the intention is to alter the program flow or to extract cryptographic keys. However, the wide-spread use of Phase Locked Loops (PLLs) prohibits the direct reach-through on the internal clock. Hence, the commonly applied procedure to induce glitches on the external clock does not have any effect on these systems. In this paper, we show by means of two different ARM Cortex-M microcontrollers, that despite the fact that the system clock is derived from the external clock signal by a PLL, fault injection by manipulation of the external clock signal is yet feasible. Even though the process of fault injection is impeded, our results indicate that the risk from this attack vector cannot be eliminated by the use of PLLs. We demonstrate this in practice by successfully performing a differential fault attack on an AES implementation.